This invention relates to organic solvent based ink formulations suitable for ink jet printing by the piezoelectric impulse drop-on-demand (DOD) method and by using several available continuous ink jet printing systems, and which contain a suitable organic solvent and one or more polyesters having one or more copolymerized near infrared fluorophores dissolved therein.
It is desirable to provide intelligible markings that are virtually invisible to the human eye on the surface of articles for identification, authentication, sorting, etc. It is known to use near infrared fluorescent compounds which have minimal light absorption in the visible light wavelength range of about 400-700 manometers (nm) and which have strong light absorbance in the near infrared wavelength region of about 700-900 nm with accompanying fluorescence to produce fluorescent radiation having wavelengths longer than the wavelength of excitation (U.S. Pat. Nos. 5,093,147; 5,336,174; 5,423,432; 5,461,136).
Aqueous ink formulations for ink jet printing which contain at least one water-dissipatable sulfopolyester/amide containing near infrared fluorophore copolymerized therein have been reported in U.S. Pat. No. 5,614,008. These aqueous ink formulations are, unfortunately, only useful for printing onto porous cellulosic materials, such as, for example paper, wood, and the like. The aqueous formulations are slow to dry and are not suitable for printing on nonporous substrates, particularly glossy paper, glass, metal foils, metal sheeting, etc. The extended drying time may further result in the printed matter becoming smeared. Another disadvantage of the aqueous formulations is when these inks are printed on nonporous substrates, the printed markings do not sufficiently resist rubbing, scraping, scuffing, etc., to remain on the printed substrate. The disclosed sulfopolyester/amides containing copolymerized infrared fluorophores are not useful in formulating organic solvent base inks for ink jet printing because of their limited solubility in organic solvents.
U.S. Pat. No. 4,540,595 discloses a water-based ink which provides markings that fluoresce when exposed to light in the near infrared wavelength. The water-based ink is used to mark documents such as bank checks for automatic identification. The dyes used to make the ink are water-soluble, cationic phenoxazines (e.g. 3,7-bis(diethylamino) phenoxazonium nitrate is the preferred fluorescent material). These dyes are not invisible. They impart blue shades to the marked substrate and the inks are not suitable for printing on nonporous substrates. The dyes have limited solubility in organic solvents utilized in ink formulations.
U.S. Pat. No. 5,093,147 discloses infrared fluorescing inks which are useful for printing invisible markings on the surface of an article. The inks use known polymethine (cyanine) laser dyes. Although the dyes used provide invisible markings, the cyanine dyes, unfortunately, have the disadvantage of fading or decomposing upon brief exposure to ultraviolet light.
U.S. Pat. No. 3,630,941 discloses certain 16,17-dialkoxyviolanthrones (also called dibenzanthrones) to be useful as infrared fluorescent markers. However, they have a high molecular weight and at room temperature, have very limited solubility in solvents normally useful for preparing solvent based inks. These compounds also have significant absorption of light having wavelengths below 700 nm and, therefore, do not usually provide invisible markings.
Japanese Laid-Open Patent Application: Hei 3-79683 (Hanada, et al.) discloses ink formulations containing certain infrared absorbing naphthalocyanine compounds useful for printing bar codes and for identifying documents to prevent falsification and forgery. Various meltable waxes (e.g. Carnauba wax) and thermoplastic resins are used as vehicles in combination with alcohols and aromatic hydrocarbons to produce thermal transfer inks.
U.S. Pat. No. 5,336,714 discloses aqueous coating compositions containing about 20 weight percent to about 35 weight percent of a water-dissipatable sulfopolyester dispersed in about 65% to about 80% by weight of water. The sulfopolyester includes 0.1 ppm by weight to about 10% by weight of a thermally stable near infrared fluorophoric compound. The coating compositions are not suitable for ink jet printing because the compositions plug the orifices of the jets.
U.S. Pat. No. 5,461,136 discloses polyesters having certain infrared fluorophores copolymerized therein to provide an invisibly marked or tagged polymeric composition. The patent also discloses a method of detection for the copolymerized polyesters. However, the polymer compositions are not adequately dissolved in the solvents normally used for solvent based inks resulting in sludge or sediment upon standing.
There is a need to provide ink formulations for printing of intelligible near infrared fluorescent markings which overcome the problems inherent in the prior art. These ink formulations need to be printable by available ink printing methods, such as an ink jet printer, at commercial speeds on a variety of substrates. The inks need to be fast drying and to provide markings which are tough enough to resist scuffing, scraping, rubbing, etc. In particular the inks should be suitable for printing on substrates such as glossy paper, glass, metal, plastics, etc.